Glass fibers are produced by drawing multiple streams of molten glass at a given rate of speed through orifices or nozzles located in a heated container known in the fiber glass industry as a bushing. The bushings containing the molten glass are electrically heated and maintained at given temperatures to provide molten glass at the orifices or nozzles at a desired viscosity. The maintenance of uniform temperature across the bushing face i.e., the area of the bushing on which the orifices or nozzles are located is important in providing uniform fiber formation from each orifice or nozzle. The fibers drawn from the orifices or nozzles are gathered after they solidify into one or more strands and are collected on a collet into one or more forming packages.
In recent years bushings have increased in size so that in industry today bushings having 800 to 2,000 or more orifices or nozzles are commonplace. It is also common practice to produce more than one strand from these larger bushings by winding, for example, two strands on a single collet using a single bushing. Typically this is accomplished by using one side of a bushing to produce one strand and the other side to produce a similar second strand. Splitting the bushing in this manner to produce more than one strand requires precise control of the bushing temperature from side to side on the bushing so that the strands produced and collected on a collet have the same yardage, i.e., the same yards per pound of glass, or viewed in another way the same weight of glass strand per package collected on the collet for a given period of time.
To provide good control of bushing temperatures on each side of a split bushing where two strands, one from each side are produced, it is also desirable to provide a control method which can be easily adjusted by an operator without recourse to the use of detailed and expensive control systems and equipment. In practice today, variations in temperature in split bushings as evidenced by a substantial variation in forming package weights on a winder used to collect two packages from a split bushing are compensated for by operators adjusting air flows to the bushing, adjusting fin cooler placement under the bushing orifices and/or terminal clamp positioning on the bushing. This system, as will be appreciated by the artisan is at best a function of operator skill which falls short of being satisfactory since such changes are manual, time-consuming, and of necessity not precise.
In U.S. Pat. No. 4,024,336, a system is described which controls two sides of a bushing by using two full wave variable impedance devices to regulate current transmitted to the bushing by a power transformer. While the relative yardage of two strands produced from a split bushing using this device may be manipulated, it has not always been effective because as change on one side of the bushing occurs it is always accompanied by a change on the other side. Further the system described requires the use of a temperature controller on each side of the bushing as well as two full wave variable impedance devices; which are costly on a per bushing basis considering that a modern fiber glass furnace forehearth can have 70 to 100 bushings positioned on it.
A need therefore exists for a simple system, easily adjustable by an operator, to control the two halves of a split fiber glass bushing producing two or more strands. The instant invention supplies that need.